We are examining the effects of mispaired bases on DNA branch migration. During homologous recombination, Holliday structures containing regions of heteroduplex DNA are formed. These heteroduplex regions can undergo branch migration which, in one direction, can increase the amount of genetic information that is exchanged between homologous chromosomes. Alternatively, net movement in the reverse direction will reduce the length of heteroduplex DNA. We are examining the role of E. coli recA protein in promoting branch migration through mispaired bases. In order to do so, we have initiated studies of nonenzymatic branch migration through mispaired bases. Using oligonucleotides to form three-stranded substrates, we observe that nonenzymatic branch migration does not proceed to any significant extent through as few as two mispaired bases (mismatches or deletions). An examination of branch migration in the presence of RecA nucleoprotein filaments formed in the presence of ATP reveals that RecA has only a very modest effect in promoting branch migration through mispaired bases. On the bases of these results, we suggest that recombinase proteins like RecA can accommodate mispaired bases in heteroduplex DNA by nucleating pairing of homologous DNAs in regions flanking mispaired bases. We are currently studying branch migration using four-stranded substrates. We are also initiating studies on the regulation of expression of the human ligase I gene. We have obtained cDNA probes corresponding to various regions of the ligase gene using Polymerase Chain Reaction (PCR) and are examining by Northern blot analysis whether the ligase gene is activated in response to UV damage in HeLa cells.